The material of optical glass to be used for an instrument utilizing light, such as a lens, is desired to have characteristics such that it has a refractive index as high as possible and a deformation point as low as possible. If the material has a high refractive index, the optical design can advantageously be made, and if the deformation point is low, a lens can be produced at a low cost by press molding. Here, the deformation point is the temperature at which expansion of glass along with the temperature rise stops and shrinkage starts, and it substantially corresponds to the temperature at which glass becomes moldable. In recent years, in addition to these characteristics, the material is desired to be composed of components containing substantially no lead with a view to facilitating e.g. disposal of waste materials.
However, many of conventional glasses for press molding have been developed on the premise that they will be used mainly in a visible light range, and no glass has been found which satisfies three conditions i.e. a no lead content, a low deformation point and a high refractive index to a light having a long wavelength, particularly a wavelength at a level of from 0.8 to 1.8 μm. Among conventional glass materials for press-molded lenses, there have been some which have reached to a level of a deformation point of 543° C. and a refractive index of about 1.88 to a wavelength of about 0.8 μm, with no lead content. However, there has been no glass material which satisfies both a refractive index higher than this and a deformation point lower than this on the premise that it contains no lead.
The first object of the present invention is to provide a glass for press molding which contains substantially no lead and which has a low deformation point and a refractive index of at least 1.9 to a light having a wavelength of 0.8 μm, and the second object is to present a lens prepared by press molding such a glass.